evolution

evolution The theory of evolution is the view that species change over time. Before Charles Darwin (1809–82), most people in the West followed Aristotle in thinking of species as immutable. Dogs could only ever give birth to dogs, and so one species could never ‘transmutate’ into another. This view was increasingly challenged during the eighteenth century, but it was not until Darwin published the Origin of Species in 1859 that the theory of evolution became widely accepted. Darwin's originality lay in proposing a mechanism by which evolution could occur — natural selection.

For a species to evolve by natural selection, three conditions must hold: (i) the members of the species must differ with respect to their chances of surviving and having offspring; (ii) these differences must be capable of being passed on to offspring; (iii) there must be occasional mutations that cause offspring to differ from their parents in ways that affect the survival chances of the offspring.

Once these conditions are in place, the species will evolve, and may in time become so different as to warrant being described as a different species.

Before Darwin, the few people who did subscribe to the theory of evolution tended to believe that the gradual change of one species into another was guided by some kind of purpose or plan. On this view, the theory of evolution was not a great threat to the idea of a divine creator. The idea that evolution occurs by means of natural selection changed all that because it assumes that the mutations which are the ultimate source of all evolutionary change are essentially random. This introduces an irreducible element of contingency into the evolutionary process, which is antithetical to any idea of a divine plan. In Darwin's theory, human beings and all other living things on this planet are, in an important sense, just accidents.

The idea that mutations are random does not mean that they are not caused. It simply means that mutations occur without any consideration for the future direction of evolution. Mutations are, so to speak, ‘blind’. Most mutations are deleterious, because for any complex organism there are far more ways of making it less effective than of improving it. These deleterious mutations are selected against. The bulk of the work of natural selection thus consists of winnowing out the bad mutations. Only occasionally does a good mutation come along, but these are retained by natural selection and over time they accumulate to produce adaptations.

Adaptations are features of organisms that show complex design and that serve (or once served) some vital function. For example, the eye is an adaptation for seeing; its complex, camera-like design is suited for that function and not any other. Before Darwin, many people argued that such complex designed features were proof of the existence of a designer, i.e. God. By showing how complex designs could emerge without the aid of a supernatural designer, Darwin demolished this argument for the existence of God.

Though Darwin's theory of evolution by natural selection was rapidly accepted by many biologists after the publication of the Origin of Species, its explanatory power was weakened by the fact that there was no satisfactory theory of heredity until the rediscovery in 1900 of a seminal paper written in 1866 by Gregor Mendel (1822–84). From Mendel's work came the idea of hereditary particles (now called ‘genes’) that were transmitted from parents to offspring and that caused the development of particular traits. This idea paved the way for the crucial distinction between genotype (the set of genes possessed by an organism) and phenotype (the physical and behavioural traits of the organism, which develop as a result of the genes interacting with the environment).

The distinction between genotype and phenotype allowed certain refinements to be made to Darwin's theory of evolution by natural selection. In the modern theory, information passes in only one direction — from the genotype to the phenotype. This is why mutations are random with respect to the direction of evolution, because the genes have no way of ‘knowing’ how best to mutate. This contrasts with the view put forward by Jean Baptiste de Lamarck (1744–1829), which Darwin himself accepted, according to which organisms could pass on to their offspring characteristics that they had acquired during their lifetime. In Lamarck's famous illustration, ancestral giraffes strenuously extended their necks to reach the leaves at the top of the trees, and their necks grew as a result of this effort. Their offspring were then born with longer necks. For this to occur, information would have to flow back from the phenotype of the adult giraffe and change the genes in some way so that the offspring would inherit genes for a longer neck. The mutations would not then be random with respect to the direction of evolution.

Twentieth-century developments in the science of genetics showed Lamarck to have been wrong. The ‘central dogma’ of modern genetics supports the view that information can only flow from the genotype to the phenotype, and not vice versa. In fact, the development of genetics was crucial to Darwinism in many other ways too. For example, the theory of population genetics, developed by Ronald Fisher (1890–1962), J. B. S. Haldane (1892–1964); and Sewall Wright (1889–1988), in the first few decades of the twentieth century, allowed evolutionary problems to be tested quantitatively. The principle achievement of these theorists was to integrate Darwinian theory and genetics into a single body of theory which is now known as ‘neo-Darwinism’, or the ‘modern synthesis’, after the title of a book by Julian Huxley, Evolution: The Modern Synthesis (1942).

In population genetics, evolution is now defined as change from one generation to the next in gene frequency. Suppose we take all the organisms in a particular population and look to see what genes are present at a given locus in the genome. On the one hand, all the organisms might have exactly the same kind of gene at that locus: in that case, there is no variation in the population at that locus, so there can be no evolution at that locus. On the other hand, we might find that half the organisms have one variant of the gene at that locus, while the other half have another variant (in technical terms, the two groups are said to have different ‘alleles’). If we then looked at the population a generation later, and found that the frequencies of the two variants had changed — for example, if only 25% of the population had the first variant, while the second variant was now found in 75% of the organisms — then and only then could evolution be said to have occurred. In fact, it would still be a case of evolution even if the change in gene frequency had no observable phenotypic effect (that is, no detectable difference between individuals with the different variants).

Natural selection is not the only means by which evolution occurs. Gene frequency can change from one generation to another as a result of other forces, such as random drift, mutation, and migration. However, unlike natural selection, these other forces cannot produce adaptations. One of the debates in contemporary evolutionary theory concerns the relative importance of natural selection vis-à-vis the other forces, such as random drift. On one side, thinkers such as George Williams and Richard Dawkins have emphasized the role of natural selection, because they are primarily interested in studying adaptations. On the other side, writers such as Stephen Jay Gould have emphasized the role of non-adaptive forces, like random drift.

Dylan Evans

Bibliography

Darwin, C. (1859). The origin of species. Penguin, Harmondsworth (1968).
Dennett, D. (1995). Darwin's dangerous idea: evolution and the meanings of life. Penguin, Harmondsworth.